Protection against electrostatic discharge (ESD) in electronic devices is generally performed using clamp circuits. These clamp circuits may be used to fix or “clamp” the positive or negative peak variations of a voltage signal to a defined value by shunting the ESD current. The timing elements of transient clamp circuits may be implemented by using resistors and capacitors (RC). Such clamp circuits are referred to as RC clamps. RC clamps may use the human body model (HBM) and the charged device model (CDM) to determine their RC time constant values. In addition, RC clamps have a number of advantages over other types of clamps including process portability, ease of simulation with standard metal oxide semiconductor field effect transistor (MOSFET) models, faster response times and lower conduction offset voltages.
As process technology continues to downscale, however, an ESD failure voltage scales down as well. For example, the failure voltages using the charged device model CDM test for ESD stressing may result in failure voltages as low as 3 volts for most advanced processes. One common method for ESD protection is using a diode in combination with an RC clamp. This approach exhibits a relatively low conduction offset voltage of about 1.6 volts, which is the sum of the diode and RC clamp offset voltages.
Unfortunately, the total offset voltage scales downward very slowly because about 1.1 volts of the total offset voltage is controlled by diode physics, which does not scale, while the RC clamp offset voltage scales very slowly because it is MOSFET threshold voltage dependent. When a protected MOSFET fails at 3 volts, this results in a very small voltage overhead for voltage (IR) drops of only 1.4 volts. If the peak CDM current is 5 amps (A), then the maximum total resistance for this case is 0.28 ohms (Ω). Thus, the sum of a diode “on” resistance, a bus resistance, and a RC clamp resistance should sum to less than 0.28 ohms. Those skilled in the art will recognize achieving the noted resistance sum is very challenging.